164 FROM NEBULA TO NEBULA 



the store which exists in the earth. This is not put forward as an 

 obvious result, it depends upon a refined dynamical theorem. 



We must clearly understand the nature of this mighty store 

 of energy from which the tides draw their power and on which 

 the moon is permitted to make large and incessant drafts. Let us 

 see in what sense the earth is said to possess a store of energy. 

 We know that the earth rotates on its axis once every day. It 

 is this rotation which is the source of the energy. Let us com- 

 pare the rotation of the earth with the rotation of the fly-wheel 

 belonging to a steam engine. The rotation of the fly-wheel is 

 really a reservoir, into which the engine pours energy at each 

 stroke of the piston. The various machines in the mill, worked 

 by the engine, merely draw upon the store of the energy accumu- 

 lated in the fly-wheel. The earth may be likened to a gigantic fly- 

 wheel detached from the engine though still connected with the 

 machines in the mill. From its stupendous dimensions and from 

 its rapid velocity, that great fly-wheel possesses an enormous 

 store of energy which must be expended before the fly-wheel 

 comes to rest. Hence it is that, though the tides are caused by the 

 moon, yet the energy they require is obtained by simply appro- 

 priating some of the vast supply available from the rotation of the 

 earth. 



It is quite true that many astronomers, without, how- 

 ever, making any pretense of logical demonstration, 

 hypothesize the existence of a statical tide, that is to say, 

 a tide on a non-rotating body. Standing by itself, this is 

 an eminently sensible idea ; but, taken in connection with 

 Newton's process of computing it, it is a wholly unwar- 

 ranted assumption. Besides, does not Young say, ' ' In 

 fact the statical theory becomes utterly unsatisfactory in 

 regard to what actually takes place "f 



Boiled down to its dregs, then, this is Newton 's Tidal 

 Theory : 



1. The moon is the primary cause of the tides, and 

 the sun the secondary. 



2. The actual, or dynamical, cause of the tides is, 

 nevertheless, neither sun nor moon, but the centrifugal 

 force of the earth 's axial rotation. In fact, the moon even 

 feeds upon the energy of this rotation. 



3. The elevation known as the earth's equatorial 

 protuberance is the running effect of that centrifugal 

 force. 



